Provide Base.length(::AbstractJuMPScalar)

```
using JuMP, Ipopt, Distances
model = Model(Ipopt.Optimizer)
@variable(model, a)
@variable(model, b)
euclidean(a, b)
```
Results in:
```
ERROR: MethodError: no method matching length(::VariableRef)

Closest candidates are:
  length(::Union{Base.KeySet, Base.ValueIterator})
   @ Base abstractdict.jl:58
  length(::Union{LinearAlgebra.Adjoint{T, S}, LinearAlgebra.Transpose{T, S}} where {T, S})
   @ LinearAlgebra /builddirs/julia/usr/share/julia/stdlib/v1.9/LinearAlgebra/src/adjtrans.jl:295
  length(::Union{SparseArrays.FixedSparseVector{Tv, Ti}, SparseArrays.SparseVector{Tv, Ti}} where {Tv, Ti})
   @ SparseArrays /builddirs/julia/usr/share/julia/stdlib/v1.9/SparseArrays/src/sparsevector.jl:95
  ...

```

Author: LebedevRI

src/JuMP.jl

@@ -1059,6 +1059,7 @@ LinearAlgebra.adjoint(scalar::AbstractJuMPScalar) = conj(scalar)
 Base.iterate(x::AbstractJuMPScalar) = (x, true)
 Base.iterate(::AbstractJuMPScalar, state) = nothing
 Base.isempty(::AbstractJuMPScalar) = false
+Base.length(::AbstractJuMPScalar) = 1
 
 # Check if two arrays of AbstractJuMPScalars are equal. Useful for testing.
 function isequal_canonical(

test/test_variable.jl

@@ -1601,4 +1601,11 @@ function test_bad_bound_types()
     return
 end
 
+function test_variable_length()
+    model = Model()
+    @variable(model, x)
+    @test length(x) == 1
+    return
+end
+
 end  # module TestVariable